1. Field of the Invention
The present invention relates to an image exposure apparatus which repeatedly scans according to the rotation of a polygon mirror a photosensitive medium with a light beam, such as a laser beam, to modulatingly expose the photosensitive medium according to an image signal. More particularly, the invention relates to an image exposure apparatus having means for correcting variation in rotation of the rotating polygon mirror.
2. Description of the Prior Art
As shown in FIG. 1, a conventional image exposure apparatus used in a color copy machine includes a rotatable cylindrical drum 50 around which are positioned a fixer/paper discharger 56, a paper supplier 51, a charger 52, a laser scanner 53, a developing agent bearer 54, and a discharger 55. The developing agent bearer 54 stores yellow 54Y, magenta 54M, cyan 54C, and black 54K developing agent. In an image exposure apparatus constructed in this way, first the paper supplier 51 supplies a sheet of photosensitive paper 57 which wraps onto the outer surface of the drum 50 with the rotation of the drum 50. Then, the sheet of photosensitive paper 57 is uniformly charged at its surface as further rotation of the drum 50 carries it past the charger 52. The charged photosensitive paper 57 is then exposed to light from the laser scanner 53 according to yellow image information of an image to be copied, forming an electrostatic latent image on the surface of the photosensitive paper 57 that represents yellow color of the image to be copied. Still further rotation of the drum carries the sheet of photosensitive paper 57 to the developing agent bearer 54 where yellow developing agent 54Y attaches to its surface where the electrostatic latent image was formed by exposure to light from the laser scanner 53. Then, the charge of the sheet of photosensitive paper 57 is removed by the discharger 55, completing the developing of the first color. This process is repeated for each color developing agent stored in the developing agent bearer 54. That is, the sheet of photosensitive paper 57 is uniformly charged, exposed to light according to the image information for each color, and developed for each color until a full color image is formed. Then the paper fixer/discharger 56 fixes the color image to the photosensitive paper 57 and discharges the photosensitive paper 57 from the color copy machine.
The positioning of components in the vicinity of the drum 50, including those of laser scanner 53, will be described in more detail while referring to FIG. 2. As is well known in the art, the laser scanner 53 disposed opposing the drum 50 includes a laser diode 40, a collimator lens 41, a polygon mirror 42, and an f-.theta. lens 43. The collimator lens 41 collates laser light produced by the laser diode 40 into a substantially parallel beam, that is, a laser beam. The polygon mirror 42 disposed in the path of the laser beam is rotated about its axis by a polygon motor 44. The rotation of the polygon mirror 42 scans the parallel beam in a main scanning direction indicated by arrow A. The polygon motor 44 outputs an FG signal containing information on its speed and angular position (phase) to a polygon motor control circuit 70. The polygon motor control circuit 70 slows down or speeds up the polygon motor 44 in accordance with a clock signal.
To a flat side of the drum 50 is provided a rotation gear engaged with a reduction gear 63 of a drum motor 61. The drum motor 61 is connected to the output of a drum motor control circuit 62. The drum motor control circuit 62 drives the drum motor 61 to rotate at a constant angular velocity by the aid of clock signals. The drum motor 61 rotates its speed reduction gear 63 which in turn causes the drum 50 to rotate. The rotation of the drum 50 moves the photosensitive paper 57 attached to the surface thereof at right angles to the scanning direction of the polygonal mirror 42 so that the surface of the photosensitive paper 57 is also scanned in an auxiliary direction indicated by arrow B. A beam detector 71 is disposed at the edge of the drum 50 where scanning in main scanning direction begins. The beam detector 71 is temporarily irradiated by the laser beam directly before scanning the region of the photosensitive paper 57 where an image is to be formed, hereinafter referred to as an image region.
The construction of the polygon motor control circuit 70 will be explained while referring to FIG. 3. The polygon motor control circuit 70 includes a speed deviation detector 46, a phase deviation detector 47, a deviation addition circuit 72, a compensator 73, and a driver 38. The FG signal from the polygon motor 44 and a clock signal are inputted to, and compared at, the phase deviation detector 47 and the speed deviation detector 46 to determine the phase deviation and the speed deviation, respectively. The phase deviation and the speed deviation are inputted to the deviation addition circuit 72, added, and then outputted as a rotation deviation signal to the compensator 73. The output from the compensator 73 is inputted to the driver 38 which applies an electric current or voltage controlled according to the output from the compensator 73 to a drive coil (not shown) of the polygon motor 44.
The polygon motor 44 is thus PLL (phase-locked loop) controlled with this polygon motor control circuit 70 to rotate the polygon mirror 42 at the predetermined rotational speed based on the clock signal. Laser light deflected by the polygon mirror 42 is collimated by the f-.theta. lens 43 into a laser beam that scans the surface of the photosensitive paper 57 on the drum 50 at a given linear speed in the main scanning direction.
Next, the construction of a conventional exposure signal processor will be explained while referring to FIG. 4. The laser diode 40 generates laser light to expose desired positions on the photosensitive paper 57 during scanning at timing controlled by the exposure signal processor based on when the beam detector 71 detects a laser beam. First, image data of an object to be copied is inputted to a D/A convertor 74 and converted into an exposure intensity signal (a) proportional to the intensity of exposure required to form a latent image. In a separate operation, the beam detector 71 generates a pulse-shaped beam detected signal (b) (hereinafter referred to as BD signal) each time it detects the laser beam at the start of each scan in the main scanning direction. The BD signal is inputted to a triangular wave generator 75 which generates a triangular wave using a crystal oscillator (not shown). The phase of the triangular wave (c) generated in the triangular wave generator 75 is adjusted to coincide with the phase of the BD signal to produce a triangular wave (c) with extremely precise and stable frequency. The exposure intensity signal (a) and the triangular wave (c) are inputted to the comparator 76, and converted into a pulse signal (d) with a duty ratio proportional to the image data. That is, areas of the image to be copied with high optical density result in a pulse signal (d) with a high duty ratio., The duty pulse signal is inputted to the laser driver 77 which causes the laser diode 40 to generate laser light accordingly. The laser beam is scanned in the main scanning direction by the rotation of the polygon mirror 42, alternately exposing and not exposing the photosensitive paper 57 according to the image data.
Modulation of the laser beam is started dependent on timing of the BD signal (generated upon detection of the laser beam by the beam detector 71 directly before scanning in the main scanning direction begins). Therefore, the photosensitive paper 57 is exposed by the laser beam during scanning in the main scanning direction at desired positions even if the polygon mirror 42 rotates with slight unevenness or if inter-surface angle or tilt of reflective surfaces on the polygon mirror 42 vary slightly.
However, there has been known a problem with conventional image exposure apparatuses in that information on the phase and rotational speed of the polygon is based on changes in the polarity of the rotor of the polygon motor, a signal (an encoder output signal) being produced accordingly, and is often imprecise because the polarization of the rotor is sometimes unclear and because only a weak signal can be obtained. Therefore regardless of how extreme the efforts to stably rotate the polygon mirror or steadily maintain the main scanning speed, because the rotation information from the polygon motor is imprecise and the polygon mirror rotates with varying speed, the laser beam exposes the photosensitive paper at imprecise positions. Rotation variation is generally about 0.02% in polygon motors controlled to rotate at a uniform speed and 0.01% even in-polygon motors using highly precise air bearings. Even mounting a highly precise speed detector to the polygon mirror to more precisely detect rotational variation does not insure reduced variation because the speed detector is often imprecisely mounted.
Because modulation of the laser beam is started according to the BD signal, the photosensitive paper can be exposed at precisely the desired positions directly after start of each scanning line in the main scanning direction. However, as scanning and modulation of the laser beam progress after the BD signal, slight variation in rotation of the polygon motor prevents accurately exposing positions toward the end of each line in the main scanning direction. Therefore providing high resolution images becomes impossible.
Further, because rotational variation differs during the charging, exposure, and developing processes for yellow tonor, magenta tonor, and cyan tonor, the colors are applied and overlapped imprecisely which can alter the color or the optical density of the copied image from that of the original.